1. Field of the Invention
This invention relates to the field of electrostatic ink jet printers, and more particularly to voltage supply means that operates to apply a print voltage between a printhead and a print substrate, to thereby selectively cause drops of ink to move on demand from selected nozzles of the printhead to the print substrate in accordance with the digital content of a print data source.
2. Description of the Related Art
The fact that liquid will deform in the presence of an electrostatic field has been known for some time. The term "Taylor cone" has been used to describe the geometric shape that results from the balance of electrostatic force, surface tension force, and internal pressure force that acts on small volume of liquid that is exposed to an electrostatic field. The electrostatic field attempts to pull atoms of the liquid out along the electrostatic field gradient, while surface tension at the same time attempts to hold the liquid in a flat state. Both of these forces are inversely proportional to the square of the radius of curvature of the liquid surface. The sharper the curvature of the liquid surface, the greater the electrostatic field attempts to pull the liquid out, and the greater the surface tension attempts to restore the liquid to a flat state. The result is a conical liquid shape having a half-angle of about 49.3-degrees, this angle being independent of liquid properties.
At the tip of an idealized Taylor cone, both of these forces become infinite. However, before this occurs in actual practice, a thin filament of liquid is drawn out of the tip of the cone along the electrostatic field gradient. It is this phenomenon that forms the basis of electrostatic, or electrohydrodynamic drop-on-demand ink jet printing, sometimes referred to as ESIJET or "easy jet".
As is well known to those of skill in the art, this ink filament does not form until the electrostatic field intensity has reached a given level. The particular level at which the ink filament forms is known to be a function of the geometry of the filament nucleation site, the physical separation between the nucleation site and the opposite electrode, and the physical and electrical properties of the ink. However, when these variables are fixed, as they are in a printer that is manufactured to a exact engineering specification, the threshold level (Et) at which an ink filament forms is constant and well behaved. Exposure of a nucleation site (i.e., an ink jet nozzle) to an electrostatic field only slightly higher in magnitude, than Et will produce an ink filament that travels in a generally straight line from the nucleation site to the opposite electrode. Exposure of a nucleation site to an electrostatic field only slightly below the magnitude Et will cause the nozzle's ink meniscus to deform, but an ink filament is not produced.
FIG. 1 represents a prior arrangement having five ink jet nucleation sites or nozzles 10-14 that are supported in a line (by means not shown) to form a linear printhead 20 that is located generally a uniform distance above a moving print substrate 15. Substrate 15 is usually nonconductive paper that moves in the Y-direction normal to the X-direction line of nozzles 10-14. In FIG. 1, nozzle 12 is selected for printing by providing a print pulse 16, of a magnitude Vp to nozzle 12 by way of conductor 3, as a bias voltage of a magnitude Vb is applied to all other nozzles.
A typical magnitude for voltage Vb for practical nozzle separation distance of about 1 mm is about 800 to about 1,200 V DC above the ground potential of plate 17. For voltage Vp, a typical magnitude is about 450 to about 800 V DC above the magnitude of voltage Vb.
Voltages Vp and Vb are applied between the respective nozzles 10-14 and the opposite electrode 17, usually a grounded metal plate. As can be seen in FIG. 1, ink filament 18 travels undeflected to paper substrate 15.
In FIG. 2, a print pulse 16 is again applied to nozzle 12, as a print pulse 19 is concomitantly applied to nozzle 13. The result is that both of the resulting ink filaments 18 and 21 are deflected from their desired points of impact on paper 15 due to the interaction of their respective electrostatic fields.
In a like manner, it is observed that when a print pulse is applied to only nozzles 11 and 13, for example, neither ink filament is significantly deflected; i.e., both filaments travel, as shown in FIG. 1. If the bias voltage and/or print voltage were to be increased and/or the same voltages were applied to more closely spaced nozzles, an electrostatic field interaction and resultant ink filament deflection would take place. However, when nozzles 10, 12 and 14 are activated, the filament that issues from nozzle 12 travels undeflected, as shown in FIG. 1, but the two ink filaments that issue from nozzles 10 and 14 are both deflected outward due to edge effects which result from the absence of a neighboring nozzle; i.e., deflected in a direction away from the adjacent nozzles 11 and 13 that have the bias voltage Vb applied thereto.
It is also observed that when a print pulse is applied to only nozzles 10, 11 and 14, for example, all three ink filaments are deflected, the ink filaments from nozzle 10 deflecting due to bias and print voltage applied to nozzle 11, and the ink filament from nozzle 14 deflecting outward as above described, as the ink filament from nozzle 11 is deflected toward non-printing nozzle 12. However, if in this situation, nozzle 12 also becomes a printing nozzle, then the ink filament that issues from nozzles 11 will not be deflected, and the ink filament that issues from nozzle 12 will be deflected. However, if in this same situation, nozzle 13 also becomes a printing nozzle, then the three ink filaments that issue from nozzles 11, 12 and 13 are not substantially deflected, but the outward deflection of the ink filaments from nozzle 10 and 14 is more pronounced due to the combination of edge effects and crosstalk.
The ink filament "crosstalk" effect is a function of electrostatic field interaction due to differences in applied voltages and, more specifically, the difference in the electrostatic field that is experienced by an ink filament nucleation site when the site is acting alone, versus the electrostatic field that this nucleation site experiences when a jetting, or print voltage Vp, is applied to one or more of its neighbor nucleation sites, or when this nucleation site has no neighbor on one or more sides. The greater the difference between the acting-alone electrostatic field and the acting-together electrostatic field, the more pronounced will be the ink filament deflection effects and ink volume differences due to crosstalk.
While the present invention will be described making reference to a linear printhead of the type shown in FIGS. 1 and 2, the invention finds utility in a more complex printhead wherein the nozzles of the printhead are arranged in a plane; for example, an X-Y matrix of ink jet nozzles. In this two-dimensional arrangement, the ink jet nozzles that are located at the border of this more complex printhead experience the same deflection characteristics as do the end nozzles 10 and 14 of FIGS. 1 and 2.